1. Field of the Invention
The present invention relates to an apparatus for separating luminance and chrominance components from a video signal and more particularly, to an apparatus for separating a luminance signal and a chrominance signal from a composite video signal in a television set (hereinafter "TV") or a video cassette recorder (hereinafter "VCR") utilizing a correlation between adjacent horizontal scanning lines.
2. Description of the Prior Art
Various types of apparatus for separating luminance and chrominance components from a video signals are known in the art.
Referring to FIG. 1, there is shown an example of conventional apparatuses for separating luminance and chrominance signals from a composite video signal in block form. Such illustrated conventional apparatus comprises a line delay 10 for delaying a composite video signal by one horizontal scanning line, a first subtracter 11 for obtaining a signal difference between adjacent horizontal scanning lines, or between one scanning line and the next scanning line, a band pass filter 12 for band pass-filtering an output signal from the subtracter 11 so as to output a chrominance signal C, a low pass filter 13 for low pass-filtering an output signal from the subtracter 11, and an absolute converter 14 for taking an absolute value of an output signal from the low pass filter 13. The illustrated prior art apparatus further comprises a hanging dot detector 15 for detecting hanging dots of steplike shape which may occur between adjacent horizontal scanning lines, a first delay 16 for delaying the composite video signal by a predetermined period of time, a second subtracter 17 for obtaining a difference between an output signal from the band pass filter 12 and an output signal from the first delay 16, a second delay 16a for delaying an output signal from the subtracter 17 by a predetermined period of time, a band reject filter 18 for rejecting a desired frequency band of the output signal from the subtracter 17, and a switch SW10 for selecting one of an output signal from the second delay 16a and an output signal from the band reject filter 18 in response to an output signal from the hanging dot detector 15, to output a luminance signal Y.
The conventional apparatus having the above-mentioned construction operates as follows.
First, the input composite video signal is delayed by one horizontal scanning line by the line delay 10. The subtracter 12 obtains a signal difference between the compositive video signal and the one line-delayed video signal from the line delay 10. Then, the output signal from the subtracter 11 is applied to the low pass filter 13 and the band pass filter 12 which band pass-filters the output signal from the subtracter 11 to output the chrominance signal C. The low frequency component from the low pass filter 13 is inputted to the absolute converter 14. At this time, if there is low correlation between adjacent horizontal scanning lines, the absolute converter 14 outputs a higher absolute value signal which causes a cross luminance signal and a cross chrominance signal to appear on a picture screen, thereby resulting in degradation of a picture quality. The cross luminance signal means the luminance signal Y which remains in the chrominance signal domain due to an erroneous separation when the composite video signal is separated into chrominance and luminance signals. On the contrary, the cross chrominance signal signifies a chrominance signal which remains in the luminance signal domain due to an erroneous separation when the composite video signal is separated into chrominance and luminance signals.
In order to solve such picture quality degradation problem caused by the higher absolute value signal from the absolute converter 14, the hanging dot detector 15 has been provided in the apparatus. The hanging dot detector 15 outputs a control signal to a switch SW10 in order to select, as the luminance signal Y, the output signal from the second delay 16a or the output signal from the band reject filter 18. If the hanging dot detector 15 outputs a high signal as the control signal to the switch SW10, then as the luminance signal Y through the switch SW10 is outputted the output signal from the band reject filter 18 which rejects a chrominance sub-carrier frequency band of the output signal from the subtracter 17. On the other hand, if the hanging dot detector 15 outputs a low signal, then as the luminance signal Y through the switch SW10 is outputted the output signal from the second delay 16a which delays the output signal from the subtracter 17 by a predetermined period of time.
However, the above-mentioned conventional apparatus suffers from a number of disadvantages, for example, low correlation between adjacent horizontal scanning lines causes a loss of high frequency horizontal component on the picture screen, resulting in degradation in a horizontal resolution.
Another chrominance and luminance separating techniques are disclosed in U.S. Pat. No. 4,240,105 to Yves C. Faroudja, entitled "Method and apparatus for separation of chrominance and luminance with adaptive comb filtering in a quadrature modulated color television system" and a report entitled "Hanging dot reducer for NTSC 1H comb filter", published Jan. 1990, RCA Engineering Laboratories, Ltd. However, these techniques do not provide a significant improvement in the resolution, either.